The invention relates generally to roller cone drill bits for drilling earth formations, and more specifically to the geometry of cutting elements on roller cone drill bits. In particular, the invention provides tooth configurations for milled tooth roller cone drill bits.
FIG. 1 shows one example of a roller cone drill bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a well bore 14. Connected to the end of the drill string 12 is roller cone-type drill bit 20, shown in further detail in FIG. 2.
Roller cone bits 20 typically comprise a bit body 22 having an externally threaded connection at one end 24, and a plurality of roller cones 26 (usually three as shown) attached at the other end of the bit body 22 and able to rotate with respect to the bit body 22. Disposed on each of the cones 26 of the bit 20 are a plurality of cutting elements 28 typically arranged in rows about the surface of the cones 26. The cutting elements 28 may comprise tungsten carbide inserts, polycrystalline diamond compacts, or milled steel teeth.
In roller cone bits, the cutting elements drill the earth formation by a combination of compressive fracturing and shearing action. Prior art milled tooth bits typically have teeth formed from steel or other easily machinable high-strength material, to which is applied in many cases a hardface overlay such as from tungsten carbide or other wear resistant materials. The hardfacing is applied by any one of a number of well known methods. There are a number of references which describe specialized exterior surface shapes for the substrate. The specialized shapes are intended to provide a cutting structure which includes more thickness of hardface overlay in wear-prone areas, so that the useful life of the teeth can be increased. Examples of such specialized substrate shapes are shown in U.S. Pat. Nos. 5,791,423, 5,351,771, 5,351,769, and 5,152,194, for example. These references show that the teeth have substantially regular trapezoidal exterior hardface surfaces. The irregular shape of the substrate outer surface is selected to provide additional hardface in the wear prone areas while maintaining a conventional exterior tooth surface.
U.S. Pat. No. 6,029,759 issued to Sue et al shows a milled tooth drill bit having teeth in a gage row (the outermost row of teeth on any cone used to maintain full drilling diameter), where these teeth have a particular outer surface. See for example FIG. 12B in Sue et al ""759. The particular outer surface of these teeth is intended to make it easier to apply hardfacing in two layers, using two different materials. The purpose of such tooth structures is to have selected hardfacing materials positioned to correspond to the level of expected wear on the various positions about the outer surface of the tooth.
The invention is a roller cone drill bit for drilling earth formations including a bit body and a plurality of roller cones attached to the bit body and able to rotate with respect to the bit body. Each of the roller cones has cutting elements disposed on it. At least one of the cutting elements includes a step shaped crest. In one embodiment, the at least one cutting element having the step shaped crest is disposed in an innermost row on one of the cones. In another embodiment, the cutting element having the step shaped crest is disposed in a row disposed between the innermost row and a gage row on one of the cones. In another embodiment, the cutting element having the step shaped crest is disposed in the gage row, wherein the axially more extensive part of the step is disposed on the gage-facing side of the cutting element.
The step shaped crest includes a more extensive segment and an axially less extensive segment along the cutting element. In a particular embodiment, the step shaped crest further includes a more extensive flank surface and a less extensive flank surface on at least one flank of the cutting element. In another embodiment, the step shape includes a more extensive segment along the crest, and two less extensive segments along the crest disposed on either side of the more extensive segment.
In a particular embodiment, the difference between the axial extent of the more extensive segment and the less extensive segment is selected to correspond to a penetration depth at which a selected earth formation is first subject to fracture-type failure during drilling.